text: There are situations where seemingly identical conformations are not equal in strain energy.  Syn-pentane strain is an example of this situation.  There are two different ways to put both of the bonds the central in n-pentane into a gauche conformation, one of which is 3 kcal mol higher in energy than the other.  When the two methyl-substituted bonds are rotated from anti to gauche in opposite directions, the molecule assumes a cyclopentane-like conformation where the two terminal methyl groups are brought into proximity.  If the bonds are rotated in the same direction, this doesn't occur.  The steric strain between the two terminal methyl groups accounts for the difference in energy between the two similar, yet very different conformations.